1. Field of the Invention
The present invention relates to signal estimation and compensation, and particularly to a method and apparatus for I/Q imbalance compensation and estimation.
2. Description of the Prior Art
In a quadrature modulation/demodulation system, the real and imaginary parts of a baseband time-domain complex signal are transmitted simultaneously from the transmitter. They are carried on two orthogonal carriers (sine and cosine waves), respectively. The receiver uses the same orthogonal carriers to demodulate the received signal and derives the original real and imaginary part of the baseband complex signal. The modulation/demodulation of the real part of the baseband complex signal is called in-phase (I) modulation/demodulation while that of the imaginary part is called quadrature-phase (Q) modulation/demodulation.
In practice, there is always a mismatch between I and Q modulation/demodulation, that is to say, there are always gain and phase offset in the I/Q modulated (or demodulated) signals. This is the I/Q imbalance known in the art.
FIGS. 1A and 1B are diagrams respectively showing the I/Q imbalance at the receiver and transmitter. As shown in the figures, crosstalk occurs due to I/Q imbalance, which can not be eliminated even with the ACG (automatic gain control) and carrier recovery circuitry. Further, ICI (inter carrier interference) also occurs for OFDM signals.
Conventionally, the solution to the previous problem is a circuitry system carefully designed to alleviate the I/Q imbalance. However, in an OFDM system, ICI is easily caused by I/Q imbalance because multi-carriers are used for high-speed transmission. This raises a need for a correction circuitry system, such as an equalizer or ICI eliminator. Even worse, in an OFDM system used for wireless LAN using burst mode transmission, the equalizer or ICI eliminator cannot achieve adequate compensation of I/Q imbalance.